This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Photosynthetic water oxidation takes place within Photosystem II (PS II), which is embedded in the thylakoid membranes of green plants, cyanobacteria and algae. Within PS II, the Mn cluster couples the one electron primary charge separation (photo-oxidation) with the four-electron water oxidation cycling through ?S-states? S0 through S4, storing oxidative equivalents serially until it reaches S4, whereupon it oxidizes two substrate water molecules to dioxygen, and returns to the S0 state. Calcium is an essential cofactor and without it the OEC cannot catalyze the oxidation of water into dioxygen, protons, and electrons. Calcium has been shown to be essential for the S2 to S3 state transition and subsequent oxygen evolution. Both XANES and EXAFS experiments have been carried out at the Mn K-edge of the OEC to determine the oxidation states and structural information about the Mn site. In comparison, there have been fewer spectroscopic investigation of the calcium cofactor. Most studies on Ca in PSII have been biochemical, because Ca has no easy spectroscopic handle. The structure of the Ca cofactor binding site has generated considerable discussion. To probe it, Ca in OEC might be substituted by Sr chemically or biochemically by growing cyanobacteria in Ca depleted Sr media, which binds at the Ca site within the protein. Under previous proposals our group has carried out Ca and Sr K-edge EXAFS experiments to resolve the question of proximity of the Ca/Sr to the Mn4-cluster of OEC. By using Ca EXAFS of the native PS II and Sr EXAFS on Sr-reactivated PS II membranes we have confirmed the proximity of the Ca/Sr at 3.5A to the Mn-cluster in the S1 dark stable state of the OEC. Using polarized Sr EXAFS on oriented Sr-reactivated samples the averaged Sr-Mn vector orientation relative to the membrane normal was determined to be within 0-23 degree. The present challenge is to determine the Mn4-Ca interactions.